The measurement of rotation angle is required in various applications, such as manual electrical switches or position detection of a motor or a valve or the like. Depending on cost and accuracy constraints, this task can be accomplished by various methods, such as mechanical contacts, potentiometers, optical encoders, or magnetic encoders.
Modern integrated circuit technology offers the possibility to integrate magnetic sensors and their readout and angle calculation electronics on a single die. This allows providing detectors of mechanical rotation which consist of a permanent magnet attached to a rotor and a monolithically integrated sensor attached to a stator, at competitive cost and good performance. The absence of mechanical contact between the rotor with the magnet and the stator with the sensor allows for hermetic encapsulation of the sensor. This permits wear-free angle measurements under harsh environmental conditions.
With the increase of compactness of electrical systems, particularly in automobiles with the arrival of hybrid engine systems, such position sensors are additionally exposed to external magnetic fields from nearby current conductors carrying strong current (e.g. more than 100 A). To maintain high sensing accuracy under such conditions, the sensor can be shielded by a ferromagnetic shield, or it must be made intrinsically robust towards such fields. This can be achieved by measuring a field gradient rather than an absolute field, since any external field is assumed constant in first approximation over the sensor as long as the sensor dimensions are small.
A sensor corresponding to this requirement is known from EP0916074B1. It describes a method and arrangement for contactless angle measurement using a magnetic field originating from a non rotation-symmetric magnet (in particular a two-pole magnet), whereby an axial field component (Bz) in parallel with the rotation axis is measured by sensor elements (so called “Horizontal Hall elements”) at several separate spots inside a plane perpendicular to the rotation axis. Then the difference between diametrically opposed sensor element values is taken, such that any signal from a constant external (disturbance) field is subtracted and it is not appearing anymore in the angle signal.
A disadvantage of the described method and arrangement is its application for small angle measurement.
US20020021124 describes a position sensor using one or more so called magnetic field concentrators (abbreviated “IMC”) to bend magnetic field lines, in combination with either horizontal Hall elements located under the IMC, or vertical Hall elements located tangentially to the edge of the IMC.